The invention relates to an adjustable intervertebral implant and expansion unit that allows continuous, as opposed to discrete, adjustment of the support of the spine.
The bones and connective tissue of an adult human spinal column consists of more than twenty discrete bones coupled sequentially to one another by a tri-joint complex which consists of an anterior disc and the two posterior facet joints, the anterior discs of adjacent bones being cushioned by cartilage spacers referred to as intervertebral discs. The spinal column of bones is highly complex in that it includes the coupled bones, housing and protecting critical elements of the nervous system having innumerable peripheral nerves and circulatory bodies in close proximity. In spite of these complications, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction.
Genetic or developmental irregularities, trauma, chronic stress, tumors, and degenerative wear are a few of the causes which can result in spinal pathologies for which surgical intervention may be necessary. A variety of systems have been disclosed in the art which achieve immobilization and/or fusion of adjacent bones by implanting artificial assemblies in or on the spinal column. The region of the back which needs to be immobilized, as well as the individual variations in anatomy, determine the appropriate surgical protocol and implantation assembly. With respect to the failure of the intervertebral disc, the interbody fusion cage has generated substantial interest because it can be implanted laparoscopically into the anterior of the spine, thus reducing operating room time, patient recovery time, and scarification.
These prior art devices generally include a tubular metal body having an external surface threading. They are inserted transverse to the axis of the spine, into preformed cylindrical holes at the junction of adjacent vertebral bodies. Two cages are generally inserted side by side with the external threading tapping into the lower surface of the vertebral bone above, and the upper surface of the vertebral bone below. The cages include holes through which the adjacent bones are to grow. Additional material, for example autogenous bone graft materials, may be inserted into the hollow interior of the cage to incite or accelerate the growth of the bone into the cage.
These cages have enjoyed clinical success in promoting fusion and grossly approximating proper disc height, however, they do have specific drawbacks which limit their effectiveness. First among these drawbacks is that the devices, once implanted, do not permit the spine to retain its original and proper curvature. Causing a fusion to grow and immobilize the spine at a curvature which is not natural can cause discomfort and potentially damaging effects.
European Patent Application No 664,994 describes an intervertebral implant. The drawback of this device is that the implant is integral with the expander unit. This device can be inserted only as a unit into the intervertebral space, so that insertion must be by knocking the implant into its site. Moreover this intervertebral implant can be expanded only discretely, whereby the pre-surgical planning of the expansion angle can be carried out only inexactly.
An intervertebral implant for insertion into an intervertebral space between adjacent vertebrae is described. The intervertebral implant comprises a yoke member having an first surface facing in a first direction and a second surface facing in a second direction. There are four legs attached to the yoke member. These legs extend in the second direction. The legs are sized and shaped to support abutting vertebrae: the first and third legs being shaped and sized to support a first vertebrae, and second and fourth legs being shaped and sized to support a second vertebrae. The first and third legs define a first vertebral plane and the second and fourth legs define a second vertebral plane, and wherein the first vertebral plane and second vertebral plane are substantially mutually parallel. The ends remote from the yoke member of the four legs being spaced apart from one another. There is a first gap defined between the first and second legs and a second gap defined between the third and fourth legs. There is a guide cylinder attached to the yoke member and extending in the second direction. The guide cylinder has a threaded surface extending at least along a portion thereof.
In one embodiment the legs are integral with the yoke. The legs and the yoke form a U shape. The guide cylinder runs between the four legs. In a preferred embodiment, the guide cylinder is hollow, and the threaded portion is on the interior, the exterior, or most preferably on both the interior and the exterior of the hollow cylinder. The intervertebral implant may have one or more guide ribs extending from the side of the legs opposite the side facing the first gap or the second gap. These guide ribs typically run substantially parallel to the guide cylinder and/or form a herringbone or a serrate contour. The legs comprise perforations to facilitate bone growth through the intervertebral implant. The legs may have one or more lips that help keep bone chips, inserted to promote bone growth, in place between the legs.
The intervertebral implant further comprises a dilator member configured to simultaneously spread the first gap and the second gap as the dilator member travels along the guide cylinder toward the yoke member. The dilator member slidingly interacts with a portion of the legs facing the first and second gaps, thereby expanding the gap between the legs. One or more grooves are preferably placed in the portion of the legs facing the first gap, the second gap, or both, that is contacted by the dilator member. These grooves are shaped and sized so that a portion of the dilator member can enter the grooves. The grooves are a predetermined distance from the yoke. In its fixed position, the dilator member is threadably connected to the guide cylinder.
A tool useful for inserting the expansion unit is an insertion bush. The insertion bush is substantially cylindrical and has a hole running along the longitudinal axis. The hole is shaped and sized to permit the guide cylinder to slide into the hole. The outer diameter on at least a portion of the insertion bush is greater than the distance between the first intervertebral and second intervertebral planes. The insertion bush has a thread on an outer surface, where the thread is adapted to contact vertebrae.